Four-wheel drive vehicles having the following configuration are known. A drive source (e.g., an engine) and a transmission are disposed in a front part of a vehicle body so that their axes extend in vehicle front-and-rear directions. A transfer device is provided for outputting torque of the drive source. The torque is transmitted from an output part of the transmission to rear wheels (main drive wheels) of the vehicle via a rearwardly-extended rear-wheel propeller shaft and a rear-wheel differential. The transfer device also extracts a portion of the output torque to be outputted to front wheels (part-time drive wheels) of the vehicle. The portion of the output torque extracted by the transfer device (front torque) is outputted to the front wheels via a forwardly-extended front-wheel propeller shaft and a front-wheel differential. Thus, in addition to the rear wheels, the front wheels are also drivable.
In this kind of four-wheel drive vehicle, a coupling for extracting the front torque is disposed in the transfer device. By completely engaging the coupling, the vehicle enters into a four-wheel drive state where the output torque is transmitted to the front and rear wheels evenly. By completely releasing the coupling, the vehicle enters into a two-wheel drive state where the output torque is transmitted only to the rear wheels. When the engagement state of the coupling is between the complete engagement and the complete release, the torque distribution to the front wheels is adjusted according to the engagement state. For example, JP2009-257432A discloses such a transfer device.
The transfer device of JP2009-257432A is provided with a coupling which extracts front torque, on an input shaft where an output torque of a drive source is transmitted from an output part of a transmission. The front torque extracted by the coupling is transmitted via a transmission mechanism to a front-wheel output shaft provided in parallel to the input shaft, and thus, front wheels are drivable in addition to rear wheels. The transmission mechanism includes a master sprocket provided on the input shaft, a slave sprocket provided on the front-wheel output shaft, and a chain wound around both sprockets.
In the four-wheel drive vehicle mounted with such a transfer device, especially when the drive source is an engine, a variation in the output torque of the engine is transmitted to the transfer device via the transmission. When the front torque, which is extracted by the coupling, is zero or relatively small, a drivetrain on the front wheels side (front-wheel drivetrain) vibrates greatly due to the drivetrain resonating with the torque variation of the engine, which may cause a teeth rattling noise between gears provided in the drivetrain.
To reduce this vibration, within an operating range where the drivetrain resonates with the torque variation of the output torque of the engine, the torque distribution to the front wheels by the coupling may be increased so as to apply a load on the drivetrain and increase the front torque. Thus, the teeth rattling noise is reduced. However, this manner results in increasing a drive loss and fuel consumption, and therefore, it is desirable to reduce the rattling noise caused by the resonance of the drivetrain, without increasing the torque distribution to the front wheels.
In this regard, a damper may be disposed between the coupling provided on the input shaft which receives an output of the transmission, and driving-side transmission members (e.g., the sprockets and gears of the transmission mechanism for outputting the torque to a part-time-drive-wheel output shaft). However, when high torque is frequently transmitted from the coupling to the transmission mechanism via the damper, an elastic member of the damper deteriorates and damper durability degrades. Although it may be considered to enhance the rigidity of the elastic member of the damper in order to improve the damper durability, the vibration of the drivetrain cannot suitably be reduced in this manner, and the problem of the teeth rattling noise cannot be solved effectively.
Therefore, the present applicant has proposed, in a prior application (JP2015-191827), an invention of a transfer device which reduces the teeth rattling noise between gears in a drivetrain without increasing fuel consumption and improves damper durability, by providing a damper between a coupling and a drive gear.
Here, the transfer device of the prior invention is described with reference to FIG. 8 accompanying the present disclosure. The transfer device of the prior invention focused on that the teeth of the gears in the drivetrain easily make the rattling noise when a front torque which is extracted by a coupling C is relatively low. The transfer device transmits a drive force to a drive gear G of a transmission mechanism via a damper D only when the torque extracted by the coupling C is low, and transmits the drive force to the drive gear G directly from the coupling C without passing through the damper D when the torque is high.
The transfer device includes the drive gear G and the coupling C on an input shaft O which receives an input from a transmission, and is also provided with the damper D between the drive gear G and the coupling C. A main body C1 (an output member of the coupling C) is provided with a first cylindrical coupling part C2 extending toward the drive gear and a second cylindrical coupling part C3 further extending from the first coupling part C2 toward the drive gear G. An outer circumferential surface of the first coupling part C2 and an inner circumferential surface of an inner cylindrical member D1 of the damper D are coupled to each other via a first spline-fitted section S1. The drive gear G is provided with an extension part G1 extending toward the coupling C, and an inner circumferential surface of the extension part G1 and an outer circumferential surface of the second coupling part C3 of the coupling C are coupled to each other via a second spline-fitted section S2.
Further, an outer cylindrical member D2 of the damper D is coupled to a drive force transmission member P. This drive force transmission member P is provided with a cylindrical part P1 extending toward the drive gear and this cylindrical part P1 is fitted to an outer circumferential surface of the extension part G1 of the drive gear G.
Here, a backlash between teeth on the outer circumferential surface of the first coupling part C2 of the coupling C and teeth on the inner circumferential surface of the inner cylindrical member D1 of the damper D, which constitute the first spline-fitted section S1, is set small. Thus, a relative rotation between the coupling C and the damper D is zero or substantially zero. On the other hand, the backlash between teeth on the outer circumferential surface of the second coupling part C3 of the coupling C and teeth on the inner circumferential surface of the extension part G1 of the drive gear which constitute the second spline-fitted section S2, is set large. Thus, a relative rotation between the coupling C and the drive gear G is allowed within a given rotational angle.
With such a structure of the transfer device, when the front torque extracted by the coupling C is lower than a given value, deformation of an elastic member D3 of the damper D is small and a relative rotation of the outer cylindrical member D2 to the inner cylindrical member D1 is small. Therefore, in the second spline-fitted section S2 with large backlash, the teeth at the extension part G1 side of the drive gear G do not contact with the teeth at the second coupling part C3 side of the coupling C, and no torque is transmitted. Therefore, the front torque is transmitted from the coupling C to the drive gear G via the first spline-fitted section S1, the damper D, and the drive force transmission member P.
On the other hand, when the front torque extracted by the coupling C is higher than the given value, the deformation of the elastic member D3 of the damper D is large and the relative rotation of the inner cylindrical member D1 to the outer cylindrical member D2 is large. Therefore, also in the second spline-fitted section S2 with large backlash, the teeth on the inner circumferential surface of the extension part G1 contact the teeth on the outer circumferential surface of the second coupling part C3, and torque is transmitted. Therefore, the front torque is transmitted from the coupling C to the drive gear G via the first spline-fitted section S1, the damper D, and the drive force transmission member P, as well as partially transmitted to the drive gear G via the second spline-fitted section S2 which is the spline-fitted section between the coupling part C3 of the coupling C and the drive gear G.
Here, the second spline-fitted section S2 functions as a stopper mechanism which restricts the relative rotation amount of the inner cylindrical member D1 to the outer cylindrical member D2 of the damper D, so that an application of torque higher than the given value to the damper D is avoided. For this, the teeth rattling noise needs to be reduced while preventing an increase in fuel consumption, and the damper durability needs to be secured.
In this regard, since the second spline-fitted section S2 is provided on the drive gear G side in the transfer device illustrated in FIG. 8, the second coupling part C3 of the coupling C needs to be extended close to the drive gear G. For this, a multi-layer structure is provided in which the second coupling part C3 is disposed between the damper D and the drive gear G to overlap, in radial directions of the transfer device, with the structure in which the outer cylindrical member D2 of the damper D and the drive gear G are coupled using the drive force transmission member P. As a result, the size of the transfer device in the radial directions may increase. Therefore, it is desired to downsize the transfer device in order to improve mountability and ride comfort of the vehicle.